1 Answer
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DPO = N_defects / (Number of units * Number of opportunities per unit).

I'll base my answer on this definition.

The number of opportunities by itself already contains information about the number of units, since

N_opportunities = Number of units * Number of opportunities per unit.

In your example, one paperclip (one unit) has a certain probability of containing a defect. There may be, say, 3 kinds of defects (bent, broken, brittle), so

N_opportunities = 1 unit * 3 defects per unit = 3 opportunities

If you found your one paperclip was bent then the DPO would be

DPO = 1 defect / 3 opportunities = 0.33 (2 decimal places)

Of course, this metric is only useful for considering samples which are sufficiently large that the probabilities of the defects manifest in a meaningful way e.g. so that expectation values start to become useful quantities.

For example, consider 2 samples (A and B), each of 10000 paperclips. If sample A had 20 bent paperclips, 3 broken paperclips, and 7 brittle paperclips from a sample of 10000 then